This invention relates to a power steering system capable of minimizing a control flow supplied to a valve mechanism during non-steering and thereby reducing energy loss.
There have been power steering systems which to reduce energy loss are constructed to decrease a control flow supplied to a valve mechanism when an assist force is not required.
Power steering systems of this kind are disclosed for example in Japanese Patent Publication No.H8-192758, Japanese Patent Publication No.H8-268304 and Japanese Patent Publication No.H8-301132, and an example of one is shown in FIG. 15.
As shown in FIG. 15, a pump is connected to a valve mechanism V by way of a flow control mechanism 1.
In the flow control mechanism 1, a variable throttle 2 is connected to the pump. This variable throttle 2 in its normal state maintains a minimum aperture, but when the pressure on its upstream side increases it switches against the resistance of a spring 3 and increases its aperture.
Also, upstream of the variable throttle 2, a flow control valve 4 is connected to the pump. This flow control valve 4 switches in correspondence with the pressure difference across the variable throttle 2 and keeps that pressure difference constant. Therefore, if the aperture of the variable throttle 2 is constant, the flow passing through the variable throttle 2 is kept constant.
A relief valve 5 determines the maximum pressure of the circuit.
A control flow controlled by the flow control mechanism 1 is supplied to a valve mechanism V. This valve mechanism V controls the control flow and distributively supplies it to a power cylinder C.
In this valve mechanism V, for example when a steering wheel W is turned in one direction, the apertures of control throttles 6a, 6c increase, and at the same time the apertures of control throttles 6b, 6d decrease. And reversely, when the steering wheel W is turned in the opposite direction, the apertures of the control throttles 6b, 6d increase and simultaneously the apertures of the control throttles 6a, 6c decrease.
In this power steering system, during non-steering, because the load pressure P of the power cylinder C is low, the pressure upstream of the variable throttle 2 is also low, and the variable throttle 2 maintains its minimum aperture. Consequently, only a minimum flow Q.sub.1 determined by this minimum aperture is supplied to the valve mechanism V side (the region a in FIG. 16).
During steering, on the other hand, the load pressure P of the power cylinder C increases, and when it reaches a predetermined pressure P.sub.1 the variable throttle 2 switches and its aperture increases. Consequently, a flow Q controlled in correspondence with the aperture of the variable throttle 2 is supplied to the valve mechanism V side (the region b in FIG. 16).
And when the load pressure P of the power cylinder C exceeds a set pressure P.sub.2, the aperture of the variable throttle 2 is held at a maximum aperture, and a flow Q.sub.2 required for an assist force is supplied to the valve mechanism V side (the region c in FIG. 16).
However, in the related art power steering system described above, in the region (the region b of FIG. 16) over which the control flow Q changes from the minimum flow Q.sub.1 to the flow Q.sub.2 required for an assist force, the pressure in the valve mechanism V also changes as a result of this change in flow.
Consequently, in that instant an assist force suddenly develops, and the driver may experience a disconcerting feeling such as one of the steering wheel being taken over, and the steering feeling thus deteriorates.